NewOS is a open source operating system with an emphasis on design and portability.
It is largely implemented in C/C++, with a small amount of assembly.
Full source is available.

Currently, the system is mostly a kernel with a minimal amount of user space libraries
and applications. Thus far, most of the work has been put into the kernel and other
underlying support. As a result the system isn't that interesting from an end-user
point of view (no gui, simple commands on a command line), but that can change given
a moderate amount of work by the maintainers or any volunteers.